In the field of ophthalmology it is known to utilize a so-called photosensitizer and electromagnetic radiation to alter the biomechanical and biochemical properties of eye tissue, namely the cornea, for example, for therapeutic purposes.
The human eye is delimited by the outer coat of the eyeball. In the rear region of the eye, the outer coat of the eye is formed by the white sclerotic coat (sclera). The cornea, which is permeable to visible light, is located in the anterior region. Deformations of the outer coat of the eye can be the cause of defective vision. For example, one form of short-sightedness, axial myopia, can result from a sclerotic axial elongation of the eye. An ellipsoidal surface of the cornea can lead to a form of astigmatism, which is referred to as keratoectasia or astigmia. Keratoconus is a further disease of the cornea. In keratoconus, an unnatural softening of the cornea leads to a progressive thinning and conical deformation of the ocular cornea. As the convexity increases, the cornea usually becomes thinner underneath the center or the highest point (apex) of the cornea. In rare cases, perforations can form in the posterior cornea, thereby allowing the fluid from the anterior chamber of the eye to enter the cornea. This is referred to as acute keratoconus, which must be treated immediately, for example, with the medical procedure known as keratoplasty.
Brillouin spectroscopy was combined with OCT (optical coherence tomography) to create a method for the biomechanical, contactless measurement of the stability of the human cornea in order to detect an early stage of keratoconus, thereby making it possible to intervene at an early stage and prevent the disease from progressing.
A subsequent application can be that of stabilizing the cornea by cross-linking. This treatment results in a photochemical, non-tissue abrading stabilization or alteration of the biomechanical and biochemical properties of the cornea. A photosensitizer solution is applied onto or into the eye tissue to be altered and is exposed to radiation that cures the photosensitizer. Electromagnetic radiation in the wavelength range from approximately 300 nm to approximately 800 nm (UV-A radiation or NIR radiation) is usually used as the primary radiation in this case.
Vitamin B2, which is also known as riboflavin, is commonly used as the photosensitizer at the present time. In original applications, the riboflavin was made viscous by the carrier medium dextran such that the epithelium of the cornea had to be removed, at least in part, in order to ensure that riboflavin penetrated the cornea. Modern compositions of active ingredients are liquid like water and have already overcome the limitations for diffusion of the riboflavin molecules into the tissue of the cornea, and therefore the painful removal of the epithelium—and the resultant pain experienced by the patient and the subsequent healing process of the epithelium—no longer appears necessary.
The objective of corneal cross-linking is that of strengthening the stability of the cornea. The main tissue of the cornea, the so-called stroma, comprises individual collagen fibers, which are connected to one another. The corneal stroma can be treated in a specific manner in order to create additional bonds between the individual collagen fibers, i.e. cross-links. In the specific treatment of the corneal stroma, it is possible, for example, to first remove the superficial protective layer, in particular the tear film, the epithelium, and Bowman's membrane (also referred to as Bowman's layer or anterior limiting lamina) using alcohol or by folding open the flap (a small cover having a hinge-type connection to the tissue) or cap (a small cover without a connection to the tissue), for example in laser in-situ keratomileusis (LASIK), apply the photosensitizer, such as riboflavin, and subsequently irradiate the corneal tissue with UVA light for approximately 30 minutes. The linking (also referred to as cross linkage) is often referred to as cross-linking. The individual fibers therefore form a “denser mesh” with one another, which increases the overall stability of the cornea.
Conventional methods for preparing an eye for the introduction of photosensitizer into the eye tissue comprise a source for laser radiation, means for guiding and focussing the laser radiation relative to the eye tissue, and a computer for controlling the aforementioned means. The computer is programmed to control the laser radiation such that the laser radiation creates at least one channel in the eye tissue that extends at least partially in the interior of the eye tissue, e.g., from the surface of the eye tissue into the interior thereof. It is therefore possible to easily introduce the photosensitizer into at least one channel in a targeted manner without the need to remove considerable portions of the epithelium for this purpose or, for example, to fold open a flap or remove a cap.